Low ceiling temperature process for a plasma reactor with heated source of a polymer-hardening precursor material

ABSTRACT

A high plasma density etch process for etching an oxygen-containing layer overlying a non-oxygen containing layer on a workpiece in a plasma reactor chamber, by providing a chamber ceiling overlying the workpiece and containing a semiconductor material, supplying into the chamber a process gas containing etchant precursor species, polymer precursor species and hydrogen, applying plasma source power into the chamber, and cooling the ceiling to a temperature range at or below about 150 degrees C. The etchant and polymer precursor species contain fluorine, and the chamber ceiling semiconductor material includes a fluorine scavenger precursor material. Preferably, the process gas includes at least one of CHF 3  and CH 2 F 2 . Preferably, the process gas further includes a species including an inert gas, such as HeH 2  or Ar. If the chamber is of the type including a heated fluorine scavenger precursor material, this material is heated to well above the polymer condensation temperature, while the ceiling is cooled. In some cases, the plasma source power applicator is an inductive antenna overlying the semiconductor ceiling, and the ceiling has a cooling/heating apparatus contacting the ceiling through semiconductor rings. The inductive antenna in this case constitutes inductive elements between adjacent ones of the semiconductor rings.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 09/008,151,filed Jan. 16, 1998, now abandoned which is a continuation-in-part ofapplication Ser. No. 08/734,797, filed Oct. 23, 1996 (now issued as U.S.Pat. No. 6,024,826), which is a continuation-in-part of application Ser.No. 08/648,254, filed May 13, 1996 (now issued as U.S. Pat. No.6,165,311), which is a continuation-in-part of application Ser. No.08/580,026, filed Dec. 20, 1995 (currently pending), which is acontinuation of application Ser. No. 08/041,796, filed Apr. 1, 1993 nowU.S. Pat. No. 5,556,501, which is a continuation of application Ser. No.07/722,340, filed Jun. 27, 1991 (now abandoned), which is acontinuation-in-part of application Ser. No. 08/503,467, filed Jul. 18,1995 (now issued as U.S. Pat. No. 5,770,099), which is a divisional ofapplication Ser. No. 08/138,060, filed Oct. 15, 1993 (now issued as U.S.Pat. No. 5,477,975), which is a continuation-in-part of application Ser.No. 08/597,577, filed Feb. 2, 1996 (now issued as U.S. Pat. No.6,077,384), which is a continuation-in-part of application Ser. No.08/521,668, filed Aug. 31, 1995 (now abandoned), which is acontinuation-in-part of application Ser. No. 08/289,336, filed Aug. 11,1994 (now abandoned), which is a continuation of application Ser. No.07/984,045, filed Dec. 1, 1992 (now abandoned). In addition, U.S.application Ser. No. 08/648,265 filed May 13, 1996 (now issued as U.S.Pat. No. 6,165,311) discloses related subject matter.

BACKGROUND OF THE INVENTION

1. Technical Field

The invention is related to a plasma reactor for processing a workpiecesuch as a semiconductor wafer with a process employing an etchselectivity-enhancing precursor material such as polymer precursorgases.

2. Background Art

1. Background Art Relating to the Parent Application:

High density RF plasma reactors for etching contact openings throughsilicon dioxide layers to underlying polysilicon conductor layers and/orto the silicon substrate of a semiconductor wafer are disclosed in theabove-referenced application by Collins et al. Ideally, such a reactorcarries out an etch process which quickly etches the overlying silicondioxide layer wherever a contact opening is to be formed, but stopswherever and as soon as the underlying polysilicon or silicon material(or other non-oxygen-containing material such as silicon nitride) isexposed, so that the process has a high oxide-to-silicon etchselectivity. Such reactors typically include a vacuum chamber, a wafersupport within the chamber, process gas flow inlets to the chamber, aplasma source coil adjacent the chamber connected to an RF power sourceusually furnishing plasma source power and another RF power sourceconnected to the wafer support usually to furnish plasma bias power. Fora silicon oxide etch process, a process gas including an etchant such asa fluorine-containing substance is introduced into the chamber. Thefluorine in the process gas freely dissociates under typical conditionsso much that the etch process attacks not only the silicon oxide layerthrough which contact openings are to be etched, but also attacks theunderlying polysilicon or silicon material as soon as it is exposed bythe etch process. Thus, a typical etch process carried out by such areactor is not the ideal process desired and has a loweroxide-to-silicon etch selectivity. As employed in this specification,the term “etch selectivity” refers to the ratio between the etch ratesof two different materials, such as silicon dioxide and silicon (eithercrystalline silicon or polycrystalline silicon hereinafter referred toas “polysilicon”). A low etch selectivity can cause punch through. Inetching shallow contact openings to intermediate polysilicon layerswhile simultaneously etching deep contact openings to the underlyingsilicon substrate, the etch process first reaches and will punch throughthe intermediate polysilicon layer before reaching the siliconsubstrate. A very high oxide-to-silicon etch selectivity is necessary toprevent the punchthrough, depending upon the ratio between the depths ofthe silicon substrate and the intermediate polysilicon layer through thesilicon oxide. For example, if (a) the deep contact opening through theoxide to the substrate is 1.0 micron deep and is to be 50% overetched,(b) the intermediate polysilicon layer is 0.4 microns deep (below thetop of the oxide layer) and (c) if not more than 0.01 microns of theintermediate polysilicon layer are to be removed (to avoidpunch-through), then an oxide-to-silicon etch selectivity of at least110:1 is required.

It is known that oxide-to-silicon etch selectivity is enhanced by apolymer film which forms more readily over silicon and polysilicon orother non-oxygen-containing layers than over silicon dioxide or otheroxygen-containing layers. In order to form such a selectivity-enhancingpolymer film, the fluorine-containing substance in the process gas is afluoro-carbon or a fluoro-hydrocarbon. Some of the fluorine in theprocess gas is consumed in chemically etching the silicon dioxide layeron the wafer. Another portion of the fluorine reacts with other speciesincluding carbon contained in the process gas to form a polymer on thesurface of the wafer. This polymer forms more rapidly and strongly onany exposed silicon and polysilicon surfaces (or othernon-oxygen-containing surfaces) than on silicon dioxide (or otheroxygen-containing surfaces), thus protecting the silicon and polysiliconfrom the etchant and enhancing etch selectivity. Etch selectivity isfurther improved by improving the strength of the polymer formed onpolysilicon surfaces. The polymer is strengthened by increasing theproportion of carbon in the polymer relative to fluorine, which can beaccomplished by decreasing the amount of free fluorine in the plasma.For this purpose, a fluorine scavenger, such as a silicon piece, can beprovided in the reactor chamber and heated to avoid being covered withpolymer and additionally to permit silicon ions, radicals and/or neutralspecies to be removed therefrom and taken into the plasma. The siliconatoms removed from the scavenger combine with some of the free fluorinein the plasma, thereby reducing the amount of fluorine available topolymerize and increasing the proportion of carbon in the polymer formedon the wafer.

While the use of a fluorine scavenger such as a heated silicon pieceinside the reactor chamber enhances etch selectivity by strengtheningthe polymer formed on the wafer, even the etch selectivity so enhancedcan be relatively inadequate for a particular application such as thesimultaneous etching of contact holes of very different depths.Therefore, it would be desireable to increase the polymer strengthbeyond that achieved by the improved scavenging technique describedabove.

Another problem is that the rate of removal of silicon from thescavenger piece required to achieve a substantial increase in polymerstrength is so great that the silicon piece is rapidly consumed and theconsequent need for its replacement exacts a price in loss ofproductivity and increased cost. Typically the scavenger piece is apiece of silicon in the reactor chamber ceiling or wall or a piece ofsilicon near the reactor chamber ceiling. The rate of removal of silicontherefrom is enhanced by applying an RF bias potential to the siliconpiece while its temperature is carefully controlled to a prevent polymerdeposition thereon and to control the rate of silicon removal therefrom.As disclosed in the above-referenced U.S. application Ser. No.08/543,067, silicon is added into the plasma by a combination of appliedRF bias and heating of the scavenger piece. The temperature controlapparatus is integrated with the silicon piece so that replacement ofthe silicon piece (e.g., a silicon ceiling) is relatively expensive. InU.S. application Ser. No. 08/597,577 referenced above, an all-siliconreactor chamber is disclosed in which the walls and ceiling are silicon,and any fluorine scavenging is done by consuming the silicon ceiling orwalls, requiring their replacement at periodic intervals with aconcomitant increase in cost of operation and decrease in productivity.Thus, not only is it desireable to increase the polymer strength but itis also desireable to decrease the rate at which silicon must be removedfrom the scavenger to achieve a desired etch selectivity.

2. Background Art Relating to the Present Application:

The reactor structure described in the detailed description below inthis specification includes a fluorine-scavenger precursor material(such as silicon or silicon carbide) either in the form of a ceiling ora disposable ring around the pedestal, or both. In order to readilyprovide a fluorine-scavenging species to the plasma, this material isheated to a desired temperature (generally above the condensationtemperature of the polymer formed from the fluorine and carbon speciesin the plasma). While this has provided significant process advantagesas described below, the continuing trend of ever smaller semiconductordevice critical dimensions and greater competition in semiconductorprice require more improvements in process performance and costeffectiveness. Specifically, it is now desired to obtain a deeper (highaspect ratio) etch depth at a faster etch rate. A deeper etch depth isrequired to meet continuing reductions in critical dimension or openingsize, while the faster etch rate is required to meet higher productionthrough-put demands for lower device cost. It is also desired to improvephotoresist facet selectivity in silicon oxide plasma etching. Such animprovement would provide a greater margin of safety against removal ofsilicon dioxide material at the edge of an opening due removal(“faceting”) of photoresist at the edge. It is further desired to bettercontrol the etch profile to obtain a 90% taper of the vertical walls ofthe etched opening. It is also desired to critical dimension varianceacross the wafer surface (e.g., center to edge) to as low as 0.05microns or less and to minimize silicon loss at the bottom of eachetched opening to 800 angstroms or less. As yet another way of reducingcosts, it is desired to reduce the cost of consumable materials in thereactor (such as the fluorine-scavenging precursor material of theceiling). The present invention described below accomplishes all of theforegoing simultaneously.

For reactors of the type lacking a silicon or silicon-carbide ringaround the pedestal as the fluorine-scavenger precursor material, theonly fluorine-scavenger precursor material is a silicon reactor ceiling.In attempting to address the problem of through-put, the etch rate couldbe increased, theoretically, by reducing the ceiling temperature toreduce fluorine scavenging, thereby increasing the fluorine content ofthe plasma as well as the fluorine content of the protective polymerdeposited on the wafer. A sufficient reduction in ceiling temperaturewould cause more polymer to deposit on the ceiling, thereby reducing thepolymer deposition rate on the wafer. With such changes, the plasmawould etch the silicon oxide layers faster and the high-fluorine contentpolymer itself would etch the silicon oxide layers, the combined effectof which is to significantly increase the rate at which silicon oxide isetched. However, this approach has not seemed feasible because the oxideetch selectivity would decrease due to the increase in fluorine contentof the polymer condensed onto non-oxygen-containing surfaces. A polymerbased upon C—F bonding is weaker than a polymer based primarily uponpure C—C bonding, so that a polymer containing more fluorine is weakerand provides less oxide-to-silicon selectivity. This latter disadvantagecould be addressed by decreasing the plasma source power in order toenhance the oxide selectivity, but it would seem such a decrease wouldreduce the etch rate, so as to offset whatever etch rate gain wasobtained by decreasing the roof temperature. Thus, a practical solutionhas not seemed to be attainable.

SUMMARY OF THE INVENTION

A high plasma density etch process for etching an oxygen-containinglayer overlying a non-oxygen containing layer on a workpiece in a plasmareactor chamber, by providing a chamber ceiling overlying the workpieceand containing a semiconductor material, supplying into the chamber aprocess gas containing etchant precursor species, polymer precursorspecies and hydrogen, applying plasma source power into the chamber, andcooling the ceiling to a temperature range at or below about 150 degreesC. The etchant and polymer precursor species contain fluorine, and thechamber ceiling semiconductor material includes a fluorine scavengerprecursor material. Preferably, the process gas includes at least one ofCHF₃ and CH₂F₂. Preferably, the process gas further includes a speciesincluding an inert gas, such as; He or Ar. If the chamber is of the typeincluding a heated fluorine scavenger precursor material, this materialis heated to well above the polymer condensation temperature, while theceiling is cooled. In some cases, the plasma source power applicator isan inductive antenna overlying the semiconductor ceiling, and theceiling has a cooling/heating apparatus contacting the ceiling throughsemiconductor rings. The inductive antenna in this case constitutesinductive elements between adjacent ones of the semiconductor rings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified cut-away side view of a plasma reactor of thetype disclosed in a first one of the co-pending applications referred toabove.

FIG. 2 is a simplified cut-away side view of a plasma reactor of thetype disclosed in a second one of the co-pending applications referredto above.

FIG. 3 is a simplified cut-away side view of a plasma reactor of thetype disclosed in a third one of the co-pending applications referred toabove.

FIG. 4A is a cut-away side view of a plasma reactor in accordance with apreferred embodiment of the present invention employing inductiveheating of an expendable polymer-hardening precursor piece.

FIG. 4B is an enlarged cross-sectional view of the workpiece processedin a working example of the embodiment of FIG. 4, illustrating themulti-layer conductor structure of the workpiece.

FIG. 4C is an enlarged view corresponding to FIG. 4A illustrating asleeve and counterbore in which an optical fiber is inserted.

FIG. 4D is an enlarged view corresponding to FIG. 4A illustrating a longwavelength optical window inside a heat transparent window.

FIG. 4E is an enlarged view corresponding to FIG. 4A illustrating a longwavelength window separate from a heat transparent window.

FIG. 5A is a graph illustrating oxide-to-silicon etch selectivity as afunction of temperature of a polymer-hardening precursor ring.

FIG. 5B is a graph illustrating radial distribution of the polysiliconetch rate in angstroms/minute at temperatures of 240° C. and 500° C.,respectively.

FIG. 6 is a cut-away view of a plasma reactor in accordance with anotherpreferred embodiment of the invention employing radiant or infraredheating of an expendable polymer-hardening precursor piece.

FIG. 7 is a cut-away view of a plasma reactor in accordance with apreferred embodiment of the invention in which an expendablepolymer-hardening precursor piece is heated in an all-semiconductorreactor chamber.

FIG. 8A is a cut-away side view of a plasma reactor in accordance with apreferred embodiment employing heated polymer-hardening precursor piecesat separate radial locations relative to the wafer being processed.

FIG. 8B corresponds to the embodiment of FIG. 8A in which the ceiling isdivided into inner and outer portions.

FIG. 9 illustrates an embodiment of the invention in which theexpendable polymer-hardening precursor piece is a removable linerabutting the cylindrical chamber side wall.

FIG. 10 is a graph illustrating the performance of a working example ofa temperature control system in a reactor embodying the invention.

FIG. 11 is a graph illustrating the closed loop response of thetemperature system whose performance is depicted in FIG. 10.

FIG. 12 is an enlarged view of a portion of the graph of FIG. 11.

FIG. 13 illustrates an embodiment corresponding to FIG. 8A but having adome-shaped ceiling.

FIG. 14 illustrates an embodiment corresponding to FIG. 8B but having adome-shaped ceiling.

FIG. 15 illustrates an embodiment corresponding to FIG. 9 but having adome-shaped ceiling.

FIG. 16 illustrates an embodiment corresponding to FIG. 8A in whichplasma source power is capacitively coupled rather than inductivelycoupled.

FIG. 17 illustrates an embodiment corresponding to FIG. 9 in whichplasma source power is capacitively coupled rather than inductivelycoupled.

FIG. 18 illustrates a modification of the embodiment of FIG. 8A in whichthe polymer-hardening precursor ring is replaced by a polymer-hardeningprecursor chamber liner over the chamber ceiling.

FIG. 19 illustrates a modification of the embodiment of FIG. 18 adoptingthe feature of FIG. 8B in which the ceiling is divided into radiallyinner and outer (disk and annular) portions.

FIG. 20 illustrates a modification of the embodiment of FIG. 18replacing the solenoid antenna coils with the flat antenna coils of theembodiment of FIG. 3.

FIG. 21 illustrates a modification of the embodiment of FIG. 20 adoptingthe feature of FIG. 8B in which the ceiling is divided into radiallyinner and outer (disk and annular) portions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

1. Description of the Parent Application:

Referring to FIG. 1, the above-referenced co-pending U.S. applicationSer. No. 08/580,026 discloses a plasma reactor chamber 10 having acylindrical side wall 12, a flat ceiling 14, a wafer support pedestal 16for supporting a workpiece 17 being processed such as a semiconductorwafer, an inductive side coil 18 wound around the cylindrical side wall12 and independent RF power sources 20, 22, 24 connected to the ceiling14, the pedestal 16 and the inductive coil 18, respectively. Inparticular, the co-pending application of Collins et al. discloses thatthe ceiling 14 may comprise silicon in order to provide a fluorinescavenger. For this purpose, RF power is applied to the silicon ceiling14 by the RF power source 20 to enhance the removal of silicontherefrom.

Referring to FIG. 2, the above-referenced co-pending application by Riceet al. discloses that the side wall 12 is quartz and provides atemperature control system for controlling the temperature of the quartzside wall 12 and the temperature of the silicon ceiling 14. Thetemperature control system includes a cooling source 30 and heatingsource 32 coupled to the quartz side wall 12 and a cooling source 34 andheating source 36 coupled to the silicon ceiling 14. Temperature sensors38, 40 coupled to the side wall 12 and ceiling 14, respectively, aremonitored by controllers 42, 44, respectively. The controller 42 governsthe cooling source 30 and heating source 32 of the quartz side wall 12while the controller 44 governs the cooling source 34 and heating source36 of the silicon ceiling. The purpose for controlling the temperatureof the silicon ceiling 14 is, at least in part, to prevent polymeraccumulation on the ceiling 14 which would otherwise prevent the ceiling14 from donating scavenger silicon to the plasma. Therefore, controller44 maintains the ceiling temperature somewhat above the polymercondensation (or polymerization) temperature, or about 170° C. dependingupon processing conditions. At the same time, the RF power source 20applies sufficient RF power to the silicon ceiling 14 in order promoteremoval of silicon from the ceiling 14 by the plasma at a sufficientrate to scavenge the desired amount of fluorine for enriching the carboncontent of the polymer formed on the wafer to enhance etch selectivity.In fact, the combination of elevated temperature and applied RF biasovercomes the energy threshold below which interaction between theplasma and the ceiling 14 causes polymer deposition and above which theinteraction causes etching of the ceiling 14.

Referring to FIG. 3, the above-referenced co-pending application Ser.No. 08/597,577 by Kenneth S. Collins discloses that both the ceiling 14and the side wall 12 are a semiconductor such as silicon and that theycan act as windows to permit inductive coupling of RF source powerthrough themselves to the plasma. For this reason, either one or boththe side coil 18 and an overhead inductor 50 may be employed and coupleRF source power to the plasma through the silicon side wall 12 and thesilicon ceiling 14. U.S. application Ser. No. 08/597,577 disclosed aplanar overhead coil and such would be suitable for carrying out thepresent invention. However, the overhead inductor 50 of the embodimentof FIG. 3 includes inner and outer solenoids 50 a, 50 b (of the typedisclosed in above-referenced co-pending U.S. application Ser. No.08/648,254, filed May 13, 1996 (now issued as U.S. Pat. No. 6,165,311)by Kenneth S. Collins et al. entitled “Inductively Coupled RF PlasmaReactor Having Overhead Solenoidal Antenna”) separately powered byindependent RF power sources 52 a, 52 b to facilitate process uniformitycontrol. Furthermore, both the ceiling 14 and side wall 12 can beemployed as separate electrodes, so that RF power is applied to thesilicon side wall 12 by a separate RF power source 54. Sufficient RFpower is applied to either or both the silicon ceiling 14 and thesilicon side wall 12 to promote removal of silicon therefrom forscavenging fluorine. The side wall 12 and ceiling 14 are preferablymaintained above the polymer condensation temperature to permit theiruse as silicon scavenger precursors and to avoid the usual requirementfor frequent chamber cleaning operations to remove polymer andassociated contaminant deposits.

A first embodiment of the present invention is a process which does notmerely enrich the carbon content of the carbon-fluorine polymer, butactually forms a different kind of polymer which more strongly adheresto the underlying silicon, polysilicon or similar non-oxygen-containingsurfaces to be protected. The result is a revolutionary improvement inetch selectivity. It is a discovery of the invention that certainmaterials in a class including silicon, silicon carbide, graphite,silicon nitride, when raised to a higher temperature range (e.g., wellabove the polymer condensation temperature) become polymer-hardeningprecursors in which they change the chemical structure of the polymer,resulting in a polymer which is far more resistant to etching than hasbeen provided in the prior art. The process is carried out bymaintaining the temperature of a polymer-hardening precursor materialinside the reactor chamber at a higher temperature range (e.g., 180° C.to 220° C. for a silicon precursor material whose potential is floatingand in any case substantially above the applicable polymer condensationtemperature). This higher temperature range varies greatly with theapplied RF bias potential on the polymer-hardening precursor materialand with the selection of the material itself.

In a second embodiment of the invention, the polymer-hardening precursormaterial is held in a maximum temperature range at which an even greaterpolymer hardness is achieved. The maximum temperature range for asilicon precursor material held at a floating potential is above 220° C.and is preferably in the range from about 300° C. to about 700° C. Thismaximum temperature range varies greatly with the RF bias applied to thepolymer-hardening precursor material. While not necessarily subscribingto any particular theory in this specification, it is felt that, in somecase but not necessarily all, at the maximum temperature range of thepolymer-hardening precursor material, the polymer-hardening precursor(e.g., silicon) material removed therefrom by the plasma bonds with thefluorine, carbon and hydrogen atoms (assuming a fluoro-hydrocarbon gasis employed) as they polymerize, the material (e.g., silicon) thus addedproviding a different kind of polymer having an optimum resistance toetching. In some cases, the polymer thus produced in this secondembodiment is distinguished by a shiny surface.

At the higher temperature range, the heated polymer-hardening precursormaterial (e.g., in the silicon ceiling 14 in the present example): (1)reduces free fluorine in the plasma by providing scavenging fluorine tothe plasma, (2) changes the relative concentrations of carbon tofluorine and hydrogen in the plasma, (3) changes the relativeconcentrations of etchant species and polymer precursor species in theplasma near the wafer surface. At the maximum temperature range, theheated polymer-hardening precursor material performs (1)-(3) above and(4) contributes polymer-hardening precursor (silicon) material into thepolymer, producing a polymer having a resistance to etching beyond whathas been heretofore attained in the art.

Efficacy of the invention is seen, for example, in the photoresistselectivity achieved by the invention, although similar improvement insilicon-to-silicon oxide selectivity is also achieved (as describedlater in this specification). When the polymer hardening precursormaterial (silicon) is heated to 300° C., sputtering effects are observedat the photoresist facets or corners of features covered by photoresist,and the etch selectivity of oxide to photoresist (the “photoresistselectivity”) is only about 3:1. If the polymer hardening precursormaterial is heated further to about 430° C., then the photoresistselectivity jumps to about 5:1, a significant improvement. If thepolymer hardening precursor material temperature is increased evenfurther to about 560° C., then the photoresist selectivity increases toabout 6:1.

Carrying out the invention in the reactor chamber of FIG. 1, 2 or 3 canbe accomplished by employing the silicon ceiling 14 or silicon side wallor skirt 12 as the polymer-hardening precursor material and increasingthe temperature of the silicon ceiling 14 of FIG. 1, 2 or 3 (and/or thesilicon side wall 12 of FIG. 3) to the requisite temperature. Thesilicon ceiling 14 (and/or silicon side wall 12), when heated to thehigher temperature range of the present invention, becomes apolymer-hardening precursor.

One problem in using the silicon ceiling 14 (or silicon side wall 12) asa fluorine scavenger precursor is that it is consumed at a ratedetermined at least in part by the amount of RF power coupled to it andmust therefore be replaced at more frequent intervals. (RF power can becoupled to the scavenger precursor either directly from an RF powergenerator or indirectly by capacitive coupling from other chambersurfaces having RF power applied directly thereto.) Because the ceiling14 (and/or side wall 12) is integrated with the temperature controlsystem described above, its replacement entails loss of productivity dueto the amount of labor required to remove and replace it, as well as acost to acquire a new silicon ceiling 14 connectable to the temperaturecontrol apparatus. The present invention solves this problem because theprocess of the invention can further include reducing the RF biasapplied by the RF power source 20 to the ceiling 14 (or reducing the RFpower applied by the RF power source 22 to the silicon side wall of FIG.3) while further increasing the silicon ceiling (and/or side wall)temperature to compensate for the reduction in RF bias power. Theadvantage of this latter feature is that the rate at which silicon isremoved from the ceiling 14 (and/or side wall 12) is reduced with thereduction in applied RF power to the ceiling. In one example, the RFpower applied by the RF power source 20 to the ceiling 14 may be reducedfour-fold while the temperature of the ceiling 14 is increased onlymoderately from about 200° C. to about 240° C. Thus, the inventionprovides dual advantages of (a) revolutionary improvement in polymerresistance to etching and (b) reduced consumption rate of the siliconmaterial in the ceiling or side wall. The increased polymer durabilityresults in an increased etch selectivity while the decreased siliconconsumption rate results in decreased cost of operation and decreasedloss in productivity.

Even though the invention permits a reduction in consumption rate of thepolymer-hardening precursor piece (e.g., the silicon ceiling), itsreplacement is nevertheless expensive and time-consuming due at least inpart to its integration with the temperature control apparatus formaintaining the requisite temperature of the piece in accordance withthe polymer-hardening process of the present invention. However, theinvention is preferably carried out with a separate cheaply fabricatedquickly replaceable polymer-hardening precursor piece, to avoidconsuming any of the integral parts of the reactor chamber such as thechamber side wall or the chamber ceiling. Such a replaceablepolymer-hardening precursor piece can be of any suitable easilyfabricated shape (e.g., planar annulus, planar ring, solid ring,cylinder, plate, and so forth) and placed at any suitable locationwithin the reactor chamber. However, in the embodiment of FIG. 4A, theexpendable polymer-hardening precursor piece is a thin planar annularring 60 of a polymer-hardening precursor material (such as silicon)surrounding a peripheral portion of the wafer pedestal 16. While thering 60 can lie in any suitable plane within the chamber, in order topermit access to the wafer by a conventional wafer transfer mechanismthe silicon ring 60 lies slightly below or nearly in the plane of thewafer 17 held on the wafer pedestal 16.

In order to eliminate any necessity of integrating or mechanicallycoupling the polymer-hardening precursor ring 60 with a directtemperature control apparatus, heating by a method other than directconduction (e.g., radiant heating or inductive heating) is preferablyemployed. A radiant heat source such as a tungsten halogen lamp or anelectric discharge lamp may be employed. A radiant or inductive heatsource can be internal—unseparated from the ring 60, or it can beexternal—separated from the ring 60 by a transmissive window forexample. In the embodiment of FIG. 4A, an external inductive heater isemployed constituting an inductive coil 62 separated from thepolymer-hardening precursor ring 60 by a window 64 of a material such asquartz which is at least nearly transparent for purposes of inductivecoupling. In order to provide the most efficient inductive heating, thepolymer-hardening precursor ring 60 is formed of silicon having asufficiently low resistivity, for example on the order of 0.01 Ω-cm. Thefollowing is an example illustrating how to select the resistivity of asilicon version of the ring 60. If: (a) the thickness T of the ring 60must be about 0.6 cm (0.25 in) for structural-mechanical purposes, (b)the inductive heater coil 62 is driven at a frequency of 1.8 Mhz, (c) anRF skin depth δ=ΓT (for example, Γ=1) is desired for optimum absorptionefficiency, and (d) the silicon ring 60 has a magnetic permeability A,then the maximum resistivity of the silicon ring 60 is given by:

ρ=δ² δ.πf.μ

which in the foregoing example is 0.029 Ω-cm. The present invention hasbeen implemented using 0.01 Ω-cm silicon. In the case of a semiconductorsuch as silicon, there is no risk of falling short of the minimumresistivity in this case and so no computation of the minimumresistivity is given here.

In a working example corresponding to the embodiment of FIG. 4A, 4000Watts of source power at 2.0 Mhz was applied to the inductive coil 18,1400 Watts of bias power at 1.8 Mhz was applied to the wafer pedestal16, process gases of CHF₃ and CO₂ were introduced into the reactorchamber at flow rates of 120 sccm and 46 sccm, respectively, while thechamber pressure was maintained at 50 Mtorr, the ceiling temperature wasmaintained at 200° C. and the side wall temperature was maintained at220° C. The polymer-hardening precursor ring 60 was crystalline siliconand reached a temperature in the range of between 240° C. and 500° C.The polymer deposited onto the silicon and polysilicon surfaces on thewafer 17 was characterized by the shinier appearance of a polymerhardened by the process of the present invention.

The semiconductor wafer 17 processed in this working example had themulti-layer conductor structure illustrated in FIG. 4B consisting of asilicon substrate 17 a, a silicon dioxide layer 17 b and a polysiliconconductor line 17 c, the etch process being facilitated by a photoresistlayer 17 d having mask openings 17 e, 17 f defining the openings 17 g,17 h etched through the silicon dioxide layer 17 b down to thepolysilicon conductor 17 c and the substrate 17 a, respectively. A veryhigh oxide-to-silicon etch selectivity is necessary to prevent thepunchthrough, depending upon the ratio between the depths of the siliconsubstrate and the intermediate polysilicon layer through the siliconoxide. For the case in which the deep contact opening 17 h through theoxide to the substrate is 1.0 micron deep and is to be 50% overetched,the intermediate contact opening 17 g to the polysilicon layer is 0.4microns deep and not more than 0.01 microns of the intermediatepolysilicon conductor layer 17 c are to be removed (to avoidpunch-through), then an oxide-to-silicon etch selectivity of at least110:1 is required.

By increasing the temperature of the silicon ring 60 over the processingof successive wafers, it was found that oxide-to-silicon etchselectivity generally increased with temperature in the mannerillustrated in the graph of FIG. 5A. The two data points A and B of FIG.5A correspond to the curves A and B of FIG. 5B illustrating radialdistribution of the polysilicon etch rate in angstroms/minute attemperatures of 240° C. and 500° C., respectively. The etchselectivities of data points A and B of FIG. 5A were computed from theoxide etch rate of 9,000 angstroms/minute observed at both temperaturesas an etch selectivity of 30:1 at 240° C. and 150:1 at 500° C. Thus,increasing the temperature to 500° C. provides a selectivity well-abovethe 110:1 minimum selectivity required in the above-given workingexample of FIG. 4B.

In the embodiment of FIG. 4A, the temperature of the polymer-hardeningprecursor ring 60 is sensed by a temperature-sensing device 66 which isnot attached to the silicon ring 60. A controller 68 governing thecurrent or power flow through the inductor 62 monitors the output of thetemperature-sensing device 66 in order to maintain the temperature ofthe polymer-hardening precursor ring 60 at the desired temperature.Preferably, the temperature-sensing device 66 is a radiant temperaturesensor which responds to radiation from the ring 60 within a particularwavelength range. Such a radiant temperature sensor may be an opticalpyrometer responsive to thermal radiation or a fluoro-optical probewhich responds to optical pulse-stimulated emission. For this purpose,the window 64 is of a material which is at least sufficientlytransmissive within the wavelength range of the sensor 66 to provide anoptical signal-to-noise ratio adequate to enable temperature control ofthe ring 60. Preferably, and in addition, the material of the window 64(over its range of operating temperatures) does not thermally radiatestrongly (relative to the radiation from the silicon ring 60) within thewavelength range of the sensor 66, so that the radiation of the window64 is practically invisible to the sensor 66 so as to not interfere withits measurement of the silicon ring temperature.

If the polymer-hardening precursor ring 60 is silicon, then onedifficulty in measuring its temperature by an optical pyrometer is thatthe thermal emissivity of silicon varies with temperature. (The siliconemissivity also happens to vary with wavelength and doping level,although it is the temperature dependence of the emissivity that isaddressed here.) One solution to this problem is to bond a small piece70 of a black-body or gray-body radiating material such as siliconnitride to the ring 60. Preferably an optical fiber 72 (indicated indashed line) is placed with one end 72 a facing a sensing portion 74 ofthe sensor 66 and the other end 72 b facing the gray-body radiator piece70 bonded to the ring 60. (If a black-body or gray-body radiatingmaterial is not added, then the long-wavelength radiation emitted by thesilicon ring 60 at lower (e.g., room) temperatures can be carried by theoptical fiber 72 provided the fiber 72 is a long wavelength materialsuch as sapphire or zinc selenide in lieu of the usual optical fibermaterial of quartz.) Since the temperature measurement by the sensor 66can be degraded by background radiation from the plasma, it is preferredto provide a counter-bore 60 a in the ring 60 to shield the opticalfiber end 72 b from background radiation (e.g., from heated chambersurfaces and from the plasma itself) without requiring any contactbetween the optical fiber 72 and the ring 60. In addition to or insteadof providing the counter-bore 60 a to shield the optical fiber end 72 bfrom plasma or background radiation, the wavelength of the sensor 66 canbe selected to lie outside the plasma emission band (4 microns to 8microns). The optical fiber 72 may be employed with or without thegray-body radiator piece 70. The window 64 passes heat to the ring 60while the optical fiber 72 passes emission from the ring 60 to thetemperature sensor 66.

If the temperature measurement is made directly of the silicon (i.e.,without the intervening gray-body radiator piece 70), then it ispreferable to use a material such as sapphire for the optical fiber 72which is highly transmissive at the emission wavelength of silicon, andto shield the optical fiber 72 with an opaque shield. Moreover, theproblem of the silicon's emissivity varying with temperature may beameliorated as shown in FIG. 4C by providing, in registration with thecounterbore 60 a, a hole 60 b which is a relatively deep and narrow witha high (e.g., 5:1) aspect ratio, the optical fiber 72 being sunk intothe counterbore 60 a to prevent background optical noise from enteringthe fiber end and an opaque shield 72 c surrounding the remainder of theoptical fiber 72. Such a deep hole may extend axially in the siliconring 60 but preferably it extends radially from the circumferential edgeof the silicon ring 60. In this implementation, virtually none of theoptical radiation from a heater lamp or from the plasma itself can enterthe optical fiber 72 to interfere with the temperature measurement.

If the ring temperature is to be sensed through the window 64 in theabsence of the optical fiber 72, then another difficulty with measuringthe temperature of the silicon ring 60 is that below 200° C. its peakthermal emission wavelength is shifted to a very long wavelength, welloutside the optical passband of typical materials such as quartz thatcan be used for the window 64. Quartz typically is transparent betweenabout 300 nm and 3 microns, while silicon's peak thermal emissionwavelength varies from 4 microns at 400° C. to 10 microns at roomtemperature. Thus, the range of directly measurable the silicon ringtemperatures is limited, since silicon below about 200° C. does notappreciably radiate within the optical passband of quartz. One solutionillustrated in FIG. 4D is to employ a small port 64 a within the quartzwindow 64, the small port 64 a being of a material which is transparentat the long wavelengths emitted by silicon at cooler temperatures downto room temperature. The small port 64 a can be sapphire or zincselenide. The radiant temperature sensor 66 would be selected to beresponsive at the longer wavelengths passed by the small port 64 a.Alternatively, instead of the small long-wavelength port 64 a within thewindow 64, a separate long-wavelength port 65 illustrated in FIG. 4Eoutside of the window 64 may be employed and may be made of sapphire orzinc selenide. The long wavelength port 65 may be replaced by itsequivalent, a long wavelength version of the optical fiber 72, such as asapphire optical fiber.

If the sensor 66 is a fluoroptical probe, then it is unaffected by thethermal emissivity of the ring 60. In this case, a fluorescent substanceor powder is immersed in the surface of a small region of the ring 60aligned with the optical fiber end 72 b. An optical pulse is appliedperiodically to the other fiber end 72 a and the resulting opticalpulse-stimulated emission from the fluorescent powder (in the ring 60)travels from the fiber end 72 b to the fiber end 72 a to be analyzed bythe sensor 66 to determine the ring temperature. A counterbore in thering 60 shields the optical fiber end 72 b from background radiation.

In order to radiantly cool the ring 60, the window 64 may be cooled byconventional means, for example by providing a cold sink for radiantcooling of the polymer-hardening precursor ring 60. In this case, therate at which the polymer-hardening precursor ring 60 is cooled is afunction of [T_(ring)]⁴−[T_(window)]⁴, where T_(ring) and T_(window) arethe absolute temperatures (Kelvin), respectively, of thepolymer-hardening precursor ring 60 and the cooled window 64. Efficientradiant cooling of the ring 60 is attained by maintaining a 200° C.temperature difference between the silicon ring 60 and the window 64,which is readily accomplished by conventional liquid or gas coolingapparatus 67 in contact with the window 64, provided the ring 60 ismaintained within the preferred temperature range of 300° C. and 700° C.However, the ring may instead be cooled using any one of a number ofconventional techniques. For example, it may be cooled in the mannerthat the wafer is typically cooled.

Whether the ring 60 is conductively cooled by a conventional cold plateor is radiantly cooled by the window 64, in some cases it may not benecessary to provide a heat source (such as the tungsten halogen lamps).Instead, heating by the plasma itself may be more than sufficient toheat the ring 60, in conjunction with the conductive or radiant cooling,to maintain stable temperature control of the ring 60 within therequisite temperature range. Thus, in an alternative embodiment, no heatsource is provided.

In the embodiment of FIG. 6, the inductive heating coil 62 is replacedby a radiant heater 80 such as a tungsten-halogen lamp or an electricdischarge lamp which emits electromagnetic radiation of a wavelengthwithin the optical transmission band of the quartz window 64 (to avoidheating the window 64) and within the absorption band of thepolymer-hardening precursor ring 60. Preferably, the wavelength of theemission from the radiant heater 80 differs from the wavelength of theemission from the polymer-hardening precursor ring 60 in order to avoidinterfering with the temperature measurement performed by the opticalpyrometer 66. However, if the optical fiber 72 is sunk into thecounterbore 60 a and if the optical fiber is completely shielded by theopaque shield 72 c that extends down to the top of the counterbore 60 a,then the radiant heater emission cannot interfere with the temperaturemeasurement, and so it is not required, in this case, that the radiantheater emission wavelength differ from the emission wavelength of the(silicon) ring 60. In fact, this is advantageous since a number ofcommercially available detectors that could be installed at the outputend of the optical fiber are more stable near the short emissionwavelength range (1-2μ) of silicon. In this case, the temperaturemeasurement is performed at shorter wavelengths so that the longwavelength port 64 a or 65 or long wavelength (e.g., sapphire) opticalfiber is not required.

To summarize the requirements for optimum radiant heating and radianttemperature sensing: (a) the material of the window 64 is highlytransmissive at the wavelength of the radiant heat source 80 and eitherthe window 64 itself or a small assigned portion of it or an opticalfiber through it is highly transmissive at the wavelength to which thetemperature sensor 66 is responsive but is not itself highly radiant atthat wavelength; (b) the polymer-hardening precursor ring 60 is highlyabsorbing at the wavelength of the radiant heater 80 and either the ring60 itself or a material embedded in or on it is radiant at thewavelength at which the sensor 66 responds; and (c) the wavelength ofthe radiant heater 80 does not coincide with the wavelength at which thesensor 66 is responsive and yet lies within the absorption spectrum ofthe polymer precursor ring 60 and outside the absorption spectrum of thewindow 64.

The foregoing requirements can be met in various ways, for example byfirst specifying the materials of the polymer-hardening precursor ring60 and then selecting a compatible material for the window 64 and thenfinally selecting the wavelengths of the radiant heater and the sensor66 by process of elimination, or else specifying the wavelengths of thesensor 66 and the radiant heater and then selecting the materials byprocess of elimination. The foregoing requirements may be relaxed to anextent depending upon the sensitivity of the temperature measurement andtemperature control precision desired.

If radiant cooling is desired, then further requirements are imposedupon the window 64: (a) in the portion of the window 64 through whichthe sensor 66 views the ring 60 the window 64 is at least nearlytransparent to the wavelength of radiation emitted by the heated ring 60(as stated above), while (b) in other portions of the window 64 not usedby the sensor 66 to view the ring 60 the window 64 has an absorptionspectrum which includes the re-radiation wavelength of the radiantlyheated polymer-hardening precursor ring 60, so as to absorb heattherefrom to provide radiant cooling.

One way of reducing the number simultaneous constraints on the materialof the window 64 is to not require the window 64 to transmit both heatto the ring 60 and the ring's thermal emission to the sensor 66. This isaccomplished by employing the small long-wavelength (zinc selenide orsapphire) port 64 a or auxiliary window 65 for exclusive use by thesensor 66 or to employ the optical fiber 72 for exclusive use by thesensor 66, in either case reducing the function of the window 64 totransmitting radiation from the heat source only. In this case, ifradiant cooling is desired, then the only other constraint on the windowis that it absorb emission from the ring 60. This is possible as long asthe emission wavelength of the radiant heat source 80 and the ringemission are of different wavelengths. Furthermore, it is preferred thatthe sensor 66 is not responsive at the wavelengths emitted by theradiant heat source 80.

In one working example, the polymer-hardening precursor ring 60 wascrystalline silicon with a resistivity of 0.01 Ω-cm and an averageemissivity between 0.3 and 0.7, the window 64 was quartz with an opticalpassband between 300 nm and 3 microns, the sensor 66 was an opticalpyrometer that sensed radiation through an optical fiber (correspondingto the optical fiber 72) in a wavelength range of 4-10 microns and theradiant heater 80 was a 3000° K. tungsten-halogen lamp with a peak poweremission wavelength range of 0.9-1.0 micron. It should be noted thatoptical pyrometry in either a visible or non-visible wavelength rangemay be employed in carrying out the invention.

Referring now to a preferred embodiment illustrated in FIG. 7, anexpendable polymer-hardening precursor ring 60 of silicon is added tothe all-semiconductor (silicon) reactor chamber of FIG. 3 and heated bya heating device 90 which may be either an induction heater or a radiantheater. Optionally, and in addition, a separate RF bias source 400 maybe connected to the ring 60 to help, along with the heating of the ring60, to maintain the ring 60 reactive with the plasma and furnishpolymer-hardening precursor material therefrom into the plasma. Theadvantage is that none of the silicon window electrodes (e.g., thesilicon side wall 12 or the silicon ceiling 14) are used to providesilicon to the polymer chemistry and therefore they need not be heatedto the elevated temperatures (e.g., over 200° C. to 700° C.) to which apolymer-hardening precursor must be heated. Moreover, the RF bias powerapplied to the silicon wall 12 and the silicon ceiling 14 need not be sohigh as to promote consumption of the silicon material thereof. In fact,it may be preferable to apply no bias to the walls and ceiling.Preferably, the temperature of the silicon ceiling 14 and of the siliconside wall 12 and the RF power applied thereto are selected to minimizeconsumption thereof by etching, sputtering or ion bombardment thereofwhile maintaining their surfaces relatively free of polymeraccumulation, to avoid frequent chamber cleaning operations. The bestmode for accomplishing this is the light deposition mode referred toearlier in this specification. In a light deposition mode carried outduring a two-minute plasma etch/polymer deposition process, the siliconwall temperature is maintained near 100° C.-150° C., and the resultingpolymer deposition thereon is sufficiently light so that it can beremoved subsequently by a 1 to 20 second exposure to a high densityoxygen plasma temporarily generated in the chamber following etching ofthe wafer. Alternatively, but not preferably, a heavy deposition modemay be employed in which the silicon chamber walls are held near roomtemperature (for example) during the etch process.

Referring to FIG. 8A, separate independently controllablepolymer-hardening precursor rings 61, 63 are placed at different radiallocations relative to the wafer to permit further compensation forradially non-uniform processing conditions. The separate outer and innerpolymer-hardening precursor rings 61, 63 of FIG. 8A are independentlycontrolled by respective inductive or radiant heaters 80 a, 80 b,respective sensors 66 a, 66 b with respective windows 64 a, 64 b andrespective temperature controllers 68 a, 68 b. In order to compensatefor processing conditions giving rise to different etch selectivities atdifferent radial locations on the wafer 17, the user may selectdifferent polymer-hardening precursor ring temperatures to be maintainedby the different temperature controllers 68 a, 68 b.

While any suitable reactor configuration may be employed in carrying outan embodiment having the separately controlled outer and innerpolymer-hardening precursor rings 61, 63, the implementation illustratedin FIG. 8A employs a reactor having a solenoidal antenna over a heatedsemiconductor window electrode ceiling of the type disclosed inabove-referenced co-pending U.S. application Ser. No. 08/648,254 byKenneth S. Collins et al. entitled “Inductively Coupled RF PlasmaReactor Having Overhead Solenoidal Antenna”. Notably, the presentlypreferred implementation of the present invention employs the reactor ofthe above-referenced co-pending application but with only the outerpolymer-hardening precursor ring 61. The reactor of the above-referencedco-pending application includes a cylindrical chamber 140 having anon-planar coil antenna 142 whose windings 144 are closely concentratedin non-planar fashion near the center axis 146 of the cylindricalchamber 140. While in the illustrated embodiment the windings 144 aresymmetrical and have an axis of symmetry coinciding with the center axisof the chamber, the invention may be carried out differently. Forexample, the windings may not be symmetrical and/or their axis ofsymmetry may not coincide with the center of the chamber or theworkpiece center. Close concentration of the windings 144 about thecenter axis 146 is accomplished by vertically stacking the windings 144in the manner of a solenoid so that they are each a minimum distancefrom the chamber center axis 146. This increases the product of current(I) and coil turns (N) near the chamber center axis 146 where the plasmaion density has been the weakest for short workpiece-to-ceiling heights.As a result, the RF power applied to the non-planar coil antenna 142produces greater induction [d/dt] [N.I] near the chamber center axis 146(relative to the peripheral regions) and therefore greater plasma iondensity in that region, so that the resulting plasma ion density is morenearly uniform despite the small workpiece-to-ceiling height. Thus, theinvention provides a way for reducing the ceiling height for enhancedplasma process performance without sacrificing process uniformity.

In order that the windings 144 be at least nearly parallel to the planeof the workpiece 156, they preferably are not wound in the usual mannerof a helix but, instead, are preferably wound so that each individualturn is parallel to the (horizontal) plane of the workpiece 156 exceptat a step or transition between turns (from one horizontal plane to thenext).

The cylindrical chamber 140 consists of a cylindrical side wall 150 anda circular ceiling 152 which can be integrally formed with the side wall150 so that the side wall 150 and ceiling 152 can constitute a singlepiece of material, such as silicon. However, the preferred embodimentillustrated FIG. 8A has the side wall 150 and ceiling 152 formed asseparate pieces. The circular ceiling 152 may be of any suitablecross-sectional shape such as planar, dome, conical, truncated conical,cylindrical or any combination of such shapes or curve of rotation.Generally, the vertical pitch of the solenoid 142 (i.e., its verticalheight divided by its horizontal width) exceeds the vertical pitch ofthe ceiling 152, even for ceilings defining 3-dimensional surfaces suchas dome, conical, truncated conical and so forth. A solenoid having apitch exceeding that of the ceiling is referred to herein as anon-conformal solenoid, meaning that, in general, its shape does notconform with the shape of the ceiling, and more specifically that itsvertical pitch exceeds the vertical pitch of the ceiling.

A pedestal 154 at the bottom of the chamber 140 supports a workpiece156, such as a semiconductor wafer, which is to be processed. Thechamber 140 is evacuated by a pump (not shown in the drawing) through anannular passage 158 to a pumping annulus 160 surrounding the lowerportion of the chamber 140. The interior of the pumping annulus may belined with a replaceable metal liner 160 a. The annular passage 158 isdefined by the bottom edge 150 a of the cylindrical side wall 150 and aplanar ring 162 surrounding the pedestal 154. Process gas is furnishedinto the chamber 140 through any one or all of a variety of gas feeds.In order to control process gas flow near the workpiece center, a centergas feed 164 a can extend downwardly through the center of the ceiling152 toward the center of the workpiece 156. In order to control gas flownear the workpiece periphery, plural radial gas feeds 164 b, which canbe controlled independently of the center gas feed 164 a, extendradially inwardly from the side wall 150 toward the workpiece periphery,or base axial gas feeds 164 c extend upwardly from near the pedestal 154toward the workpiece periphery, or ceiling axial gas feeds 164 d canextend downwardly from the ceiling 152 toward the workpiece periphery.Etch rates at the workpiece center and periphery can be adjustedindependently relative to one another to achieve a more radially uniformetch rate distribution across the workpiece by controlling the processgas flow rates toward the workpiece center and periphery through,respectively, the center gas feed 164 a and any one of the outer gasfeeds 164 b-d. This feature of the invention can be carried out with thecenter gas feed 164 a and only one of the peripheral gas feeds 164 b-d.

The solenoidal coil antenna 142 is wound around a housing 166surrounding the center gas feed 164. A plasma source RF power supply 168is connected across the coil antenna 142 and a bias RF power supply 170is connected to the pedestal 154.

Confinement of the overhead coil antenna 142 to the center region of theceiling 152 leaves a large portion of the top surface of the ceiling 152unoccupied and therefore available for direct contact with temperaturecontrol apparatus including, for example, plural radiant heaters 172such as tungsten halogen lamps and a water-cooled cold plate 174 whichmay be formed of copper or aluminum for example, with coolant passages174 a extending therethrough. Preferably the coolant passages 174 acontain a coolant of a known variety having a high thermal conductivitybut a low electrical conductivity, to avoid electrically loading downthe antenna or solenoid 142. The cold plate 174 provides constantcooling of the ceiling 152 while the maximum power of the radiantheaters 172 is selected so as to be able to overwhelm, if necessary, thecooling by the cold plate 174, facilitating responsive and stabletemperature control of the ceiling 152. The large ceiling areairradiated by the heaters 172 provides greater uniformity and efficiencyof temperature control. (It should be noted that radiant heating is notnecessarily required in carrying out the invention, and the skilledworker may choose to employ an electric heating element instead, as willbe described later in this specification.) If the ceiling 152 issilicon, as disclosed in co-pending U.S. application Ser. No. 08/597,577filed Feb. 2, 1996 by Kenneth S. Collins et al., then there is asignificant advantage to be gained by thus increasing the uniformity andefficiency of the temperature control across the ceiling. Specifically,where a polymer precursor and etchant precursor process gas (e.g., afluorocarbon gas) is employed and where the etchant (e.g., fluorine)must be scavenged, the rate of polymer deposition across the entireceiling 152 and/or the rate at which the ceiling 152 furnishes afluorine etchant scavenger material (silicon) into the plasma is bettercontrolled by increasing the contact area of the ceiling 152 with thetemperature control heater 172. The solenoid antenna 142 increases theavailable contact area on the ceiling 152 because the solenoid windings144 are concentrated at the center axis of the ceiling 152.

The increase in available area on the ceiling 152 for thermal contact isexploited in a preferred implementation by a highly thermally conductivetorus 175 (formed of a ceramic such as aluminum nitride, aluminum oxideor silicon nitride or of a non-ceramic like silicon either lightly dopedor undoped) whose bottom surface rests on the ceiling 152 and whose topsurface supports the cold plate 174. One feature of the torus 175 isthat it displaces the cold plate 174 well-above the top of the solenoid142. This feature substantially mitigates or nearly eliminates thereduction in inductive coupling between the solenoid 142 and the plasmawhich would otherwise result from a close proximity of the conductiveplane of the cold plate 174 to the solenoid 142. In order to preventsuch a reduction in inductive coupling, it is preferable that thedistance between the cold plate 174 and the top winding of the solenoid142 be at least a substantial fraction (e.g., one half) of the totalheight of the solenoid 142. Plural axial holes 175 a extending throughthe torus 175 are spaced along two concentric circles and hold theplural radiant heaters or lamps 172 and permit them to directlyirradiate the ceiling 152. For greatest lamp efficiency, the holeinterior surface may be lined with a reflective (e.g., aluminum) layer.The ceiling temperature is sensed by a sensor such as a thermocouple 176extending through one of the holes 175 a not occupied by a lamp heater172. For good thermal contact, a highly thermally conductive elastomer173 such as silicone rubber impregnated with boron nitride is placedbetween the ceramic torus 175 and the copper cold plate 174 and betweenthe ceramic torus 175 and the silicon ceiling 152.

In the embodiment of FIG. 8A, the chamber 140 may be anall-semiconductor chamber, in which case the ceiling 152 and the sidewall 150 are both a semiconductor material such as silicon. Controllingthe temperature of, and RF bias power applied to, either the ceiling 152or the wall 150 regulates the extent to which it furnishes fluorinescavenger precursor material (silicon) into the plasma or,alternatively, the extent to which it is coated with polymer. Thematerial of the ceiling 152 is not limited to silicon but may be, in thealternative, silicon carbide, silicon dioxide (quartz), silicon nitrideor a ceramic.

The chamber wall or ceiling 150, 152 need not be used as the source of afluorine scavenger material. Instead, a disposable silicon member can beplaced inside the chamber 140 and maintained at a sufficiently hightemperature to prevent polymer condensation thereon and permit siliconmaterial to be removed therefrom into the plasma as fluorine scavengingmaterial. In this case, the wall 150 and ceiling 152 need notnecessarily be silicon, or if they are silicon they may be maintained ata temperature (and/or RF bias) near or below the polymer condensationtemperature (and/or a polymer condensation RF bias threshold) so thatthey are coated with polymer from the plasma so as to be protected frombeing consumed. While the disposable silicon member may take anyappropriate form, in the embodiment of FIG. 8A the disposable orexpendable silicon member is an annular ring 162 surrounding thepedestal 154. Preferably, the annular ring 162 is high purity siliconand may be doped to alter its electrical or optical properties. In orderto maintain the silicon ring 162 at a sufficient temperature to ensureits favorable participation in the plasma process (e.g., itscontribution of silicon material into the plasma for fluorinescavenging), plural radiant (e.g., tungsten halogen lamp) heaters 177arranged in a circle under the annular ring 162 heat the silicon ring162 through a quartz window 178. As described in the above-referencedco-pending application, the heaters 177 are controlled in accordancewith the measured temperature of the silicon ring 162 sensed by atemperature sensor 179 which may be a remote sensor such as an opticalpyrometer or a fluoro-optical probe. The sensor 179 may extend partiallyinto a very deep hole 162 a in the ring 162, the deepness and narrownessof the hole tending at least partially to mask temperature-dependentvariations in thermal emissivity of the silicon ring 162, so that itbehaves more like a gray-body radiator for more reliable temperaturemeasurement.

As described in U.S. application Ser. No. 08/597,577 referred to above,an advantage of an all-semiconductor chamber is that the plasma is freeof contact with contaminant producing materials such as metal, forexample. For this purpose, plasma confinement magnets 180, 182 adjacentthe annular opening 158 prevent or reduce plasma flow into the pumpingannulus 160. To the extent any polymer precursor and/or active speciessucceeds in entering the pumping annulus 160, any resulting polymer orcontaminant deposits on the replaceable interior liner 160 a may beprevented from re-entering the plasma chamber 140 by maintaining theliner 160 a at a temperature significantly below the polymercondensation temperature, for example, as disclosed in the referencedco-pending application.

A wafer slit valve 184 through the exterior wall of the pumping annulus160 accommodates wafer ingress and egress. The annular opening 158between the chamber 140 and pumping annulus 160 is larger adjacent thewafer slit valve 184 and smallest on the opposite side by virtue of aslant of the bottom edge of the cylindrical side wall 150 so as to makethe chamber pressure distribution more symmetrical with anon-symmetrical pump port location.

Maximum inductance near the chamber center axis 146 is achieved by thevertically stacked solenoidal windings 144.

A second outer vertical stack or solenoid 1120 of windings 1122 at anouter location (i.e, against the outer circumferential surface of thethermally conductive torus 175) is displaced by a radial distance δRfrom the inner vertical stack of solenoidal windings 144. Note thatconfinement of the inner solenoidal antenna 142 to the center and theouter solenoidal antenna 1120 to the periphery leaves a large portion ofthe top surface of the ceiling 152 available for direct contact with thetemperature control apparatus 172, 174, 175. An advantage is that thelarger surface area contact between the ceiling 152 and the temperaturecontrol apparatus provides a more efficient and more uniform temperaturecontrol of the ceiling 152.

For a reactor in which the side wall and ceiling are formed of a singlepiece of silicon for example with an inside diameter of 12.6 in (32 cm),the wafer-to-ceiling gap is 3 in (7.5 cm), and the mean diameter of theinner solenoid was 3.75 in (9.3 cm) while the mean diameter of the outersolenoid was 11.75 in (29.3 cm) using {fraction (3/16)} in diameterhollow copper tubing covered with a 0.03 thick teflon insulation layer,each solenoid consisting of four turns and being 1 in (2.54 cm) high.The outer stack or solenoid 1120 is energized by a second independentlycontrollable plasma source RF power supply 196. The purpose is to permitdifferent user-selectable plasma source power levels to be applied atdifferent radial locations relative to the workpiece or wafer 56 topermit compensation for known processing non-uniformities across thewafer surface, a significant advantage. In combination with theindependently controllable center gas feed 164 a and peripheral gasfeeds 164 b-d, etch performance at the workpiece center may be adjustedrelative to etch performance at the edge by adjusting the RF powerapplied to the inner solenoid 142 relative to that applied to the outersolenoid 190 and adjusting the gas flow rate through the center gas feed164 a relative to the flow rate through the outer gas feeds 164 b-d.While the present invention solves or at least ameliorates the problemof a center null or dip in the inductance field as described above,there may be other plasma processing non-uniformity problems, and thesecan be compensated in the versatile embodiment of FIG. 8A by adjustingthe relative RF power levels applied to the inner and outer antennas.For effecting this purpose with greater convenience, the respective RFpower supplies 168, 196 for the inner and outer solenoids 142, 190 maybe replaced by a common power supply 197 a and a power splitter 197 bwhich permits the user to change the relative apportionment of powerbetween the inner and outer solenoids 142, 190 while preserving a fixedphase relationship between the fields of the inner and outer solenoids142, 190. This is particularly important where the two solenoids 142,190 receive RF power at the same frequency. Otherwise, if the twoindependent power supplies 168, 196 are employed, then they may bepowered at different RF frequencies, in which case it is preferable toinstall RF filters at the output of each RF power supply 168, 196 toavoid off-frequency feedback from coupling between the two solenoids. Inthis case, the frequency difference should be sufficient to time-averageout coupling between the two solenoids and, furthermore, should exceedthe rejection bandwidth of the RF filters. A preferred mode is to makeeach frequency independently resonantly matched to the respectivesolenoid, and each frequency may be varied to follow changes in theplasma impedance (thereby maintaining resonance) in lieu of conventionalimpedance matching techniques. In such implementations, the frequencyranges of the two solenoids should be mutually exclusive. Alternatively,the two solenoids are driven at the same RF frequency and in this caseit is preferable that the phase relationship between the two be such asto cause constructive interaction or superposition of the fields of thetwo solenoids. Generally, this requirement will be met by a zero phaseangle between the signals applied to the two solenoids if they are bothwound in the same sense. Otherwise, if they are oppositely wound, thephase angle is preferably 180°. In any case, coupling between the innerand outer solenoids can be minimized or eliminated by having arelatively large space between the inner and outer solenoids 142, 190,as will be discussed below in this specification.

The range attainable by such adjustments is increased by increasing theradius of the outer solenoid 910 to increase the spacing between theinner and outer solenoids 142, 190, so that the effects of the twosolenoids 142, 190 are more confined to the workpiece center and edge,respectively. This permits a greater range of control in superimposingthe effects of the two solenoids 142, 190. For example, the radius ofthe inner solenoid 142 should be no greater than about half theworkpiece radius and preferably no more than about a third thereof. (Theminimum radius of the inner solenoid 142 is affected in part by thediameter of the conductor forming the solenoid 142 and in part by theneed to provide a finite non-zero circumference for an arcuate—e.g.,circular—current path to produce inductance.) The radius of the outercoil 190 should be at least equal to the workpiece radius and preferably1.5 or more times the workpiece radius. With such a configuration, therespective center and edge effects of the inner and outer solenoids 142,190 are so pronounced that by increasing power to the inner solenoid thechamber pressure can be raised into the hundreds of Mt while providing auniform plasma, and by increasing power to the outer solenoid 190 thechamber pressure can be reduced to on the order of 0.01 Mt whileproviding a uniform plasma. Another advantage of such a large radius ofthe outer solenoid 190 is that it minimizes coupling between the innerand outer solenoids 142, 190.

In the embodiment of FIG. 8A, the ceiling 152 and side wall 150 areseparate semiconductor (e.g., silicon) pieces insulated from one anotherhaving separately controlled RF bias power levels applied to them fromrespective RF sources 1210, 1212 to enhance control over the center etchrate and selectivity relative to the edge. As set forth in greaterdetail in above-referenced U.S. application Ser. No. 08/597,577 filedFeb. 2, 1996 by Kenneth S. Collins et al., the ceiling 152 may be asemiconductor (e.g., silicon) material doped so that it will act as anelectrode capacitively coupling the RF bias power applied to it into thechamber and simultaneously as a window through which RF power applied tothe solenoid 142 may be inductively coupled into the chamber. Theadvantage of such a window-electrode is that an RF potential may beestablished directly over the wafer (e.g., for controlling ion energy)while at the same time inductively coupling RF power directly over thewafer. This latter feature, in combination with the separatelycontrolled inner and outer solenoids 142, 190 and center and peripheralgas feeds 164 a, 164 b greatly enhances the ability to adjust variousplasma process parameters such as ion density, ion energy, etch rate andetch selectivity at the workpiece center relative to the workpiece edgeto achieve an optimum uniformity. In this combination, pressure and/orgas volume through individual gas feeds is individually and separatelycontrolled to achieve such optimum uniformity of plasma processparameters.

The lamp heaters 172 may be replaced by electric heating elements. Thedisposable silicon member is an annular ring 62 surrounding the pedestal54. Preferably, the annular ring 62 is high purity silicon and may bedoped to alter its electrical or optical properties. In order tomaintain the silicon ring 62 at a sufficient temperature to ensure itsfavorable participation in the plasma process (e.g., its contribution ofsilicon material into the plasma for fluorine scavenging), pluralradiant (e.g., tungsten halogen lamps) heaters 177 arranged in a circleunder the annular ring 162 heat the silicon ring 162 through a quartzwindow 178. As described in the above-referenced co-pending application,the heaters 177 are controlled in accordance with the measuredtemperature of the silicon ring 162 sensed by a temperature sensor 179which may be a remote sensor such as an optical pyrometer or afluoro-optical probe. The sensor 179 may extend partially into a verydeep hole 162 a in the ring 162, the deepness and narrowness of the holetending at least partially to mask temperature-dependent variations inthermal emissivity of the silicon ring 162, so that it behaves more likea gray-body radiator for more reliable temperature measurement.

FIG. 8B illustrates another variation in which the ceiling 152 itselfmay be divided into an inner disk 152 a and an outer annulus 152 belectrically insulated from one another and separately biased byindependent RF power sources 1214, 1216 which may be separate outputs ofa single differentially controlled RF power source.

In accordance with an alternative embodiment, a user-accessible centralcontroller 1300 shown in FIGS. 8A and 8B, such as a programmableelectronic controller including, for example, a conventionalmicroprocessor and memory, is connected to simultaneously control gasflow rates through the central and peripheral gas feeds 164 a, 164, RFplasma source power levels applied to the inner and outer antennas 142,190 and RF bias power levels applied to the ceiling 152 and side wall150 respectively (in FIG. 8A) and the RF bias power levels applied tothe inner and outer ceiling portions 152 a, 152 b (in FIG. 8B),temperature of the ceiling 152 and the temperature of the silicon ring162. A ceiling temperature controller 1218 governs the power applied bya lamp power source 1220 to the heater lamps 172 by comparing thetemperature measured by the ceiling temperature sensor 176 with adesired temperature known to the controller 1300. The master controller1300 governs the desired temperatures of the temperature controllers1218 and 68 a, 68 b, the RF power levels of the solenoid power sources168, 196, the RF power levels of the bias power sources 1210, 1212 (FIG.8A) or 1214, 1216 (FIG. 8B), the wafer bias level applied by the RFpower source 170 and the gas flow rates supplied by the various gassupplies (or separate valves) to the gas inlets 164 a-d. The key tocontrolling the wafer bias level is the RF potential difference betweenthe wafer pedestal 154 and the ceiling 152. Thus, either the pedestal RFpower source 170 or the ceiling RF power source 1212 may be simply ashort to RF ground. With such a programmable integrated controller, theuser can easily optimize apportionment of RF source power, RF bias powerand gas flow rate between the workpiece center and periphery to achievethe greatest center-to-edge process uniformity across the surface of theworkpiece (e.g., uniform radial distribution of etch rate and etchselectivity). Also, by adjusting (through the controller 1300) the RFpower applied to the solenoids 142, 190 relative to the RF powerdifference between the pedestal 154 and ceiling 152, the user canoperate the reactor in a predominantly inductively coupled mode or in apredominantly capacitively coupled mode.

While the various power sources connected in FIG. 8A to the solenoids142, 190, the ceiling 152, side wall 150 (or the inner and outer ceilingportions 152 a, 152 b as in FIG. 8B) have been described as operating atRF frequencies, the invention is not restricted to any particular rangeof frequencies, and frequencies other than RF may be selected by theskilled worker in carrying out the invention.

In a preferred embodiment of the invention, the high thermalconductivity spacer 175, the ceiling 152 and the side wall 150 areintegrally formed together from a single piece of crystalline silicon.

While the expendable polymer-hardening precursor piece has beendescribed as a planar ring 60 in the wafer support pedestal top surfaceplane, it may be of any shape and at any location, provided that it isnot too distant from the heat source to be efficiently heated therebyand provided that it shields the plasma processing region of the chamberfrom the heat source (so as to avoid diversion of power from the heatsource to plasma generation in the chamber). In the preferred embodimentsome shielding of the plasma from energy of the heat source (in additionto that provided by the silicon ring 60 itself) is provided by a pair ofring magnets 100, 102 that prevent plasma flow between the plasmaprocessing region of the chamber and the pumping annulus.

Another embodiment that provides the requisite shielding and closeproximity of the expendable polymer-hardening precursor piece to aremote heat source is illustrated in FIG. 9 in which the expendablepiece is a cylindrical silicon liner 110 abutting the cylindricalchamber side wall interior surface. A peripheral heat source 115adjacent the outside of the cylindrical side wall heats the siliconliner 110 through the side wall. The peripheral heat source 115 may bean induction heater, in which case the cylindrical chamber wall iseither an insulator such as quartz or is a semiconductor such as siliconof sufficiently high resistivity to minimize absorption and maximizetransmission of the induction field of the heat source 115 to the liner110. Alternatively, the peripheral heat source 115 is a radiant heatersuch as a tungsten halogen lamp or an electric discharge lamp. Atemperature sensor 166 and temperature controller 168 governingoperation of the peripheral heater 115 perform temperature control inthe manner of the sensor 66 and controller 68 of the embodiment of FIG.8A. In the embodiment of FIG. 9, the master controller 300 governs thetemperature controller 168.

The graph of FIG. 10 illustrates the performance of a temperaturecontrol system implemented in the embodiment of FIG. 6. The horizontalaxis is the steady state temperature in degrees Celsius at which thetemperature controller 68 has been commanded to hold the silicon ring60, while the vertical axis is the applied power in Watts required tomaintain the selected ring temperature. The graph of FIG. 11 illustratesthe closed loop temperature response of the system of FIG. 6, thehorizontal axis being time in seconds and the vertical axis being thering temperature in degrees Celsius. In the graph of FIG. 11, the ring60 begins at an initial temperature near room temperature and, afterabout 30 seconds, the controller 68 is commanded to put the ringtemperature at 440° C. This temperature is reached at about 310 secondswith no overshoot and only a trace amount of system noise. At about 550seconds a plasma is ignited in the chamber and is extinguished at about1000 seconds, the effect on the ring temperature being almostunobservable in FIG. 11. This latter event proves the stability of thetemperature control system and its responsiveness. FIG. 12 is a greatlyenlarged view of a portion of the graph of FIG. 11 in the neighborhoodof the time window from 301 seconds (when the target temperature isreached with no overshoot) and including 550 seconds (when the plasma istemporarily turned on). The plasma was ignited with 3.2 Kwatts of sourcepower at 550 seconds and the enlarged view of FIG. 12 reveals a shortspike in the ring temperature coincident with that event. This data wasobtained using the fluoro-optical probe version of the sensor 66 and theoptical fiber 72.

In lieu of heating the polymer hardening precursor piece (e.g., thesilicon ring 60), RF bias power may be applied from a source 400(indicated in dashed line in FIG. 7) to the piece to achieve a desiredeffect in increasing polymer resistance to etching. The skilled workercan readily ascertain the requisite RF bias power level for thisapplication by increasing the RF bias power on the piece (e.g., thesilicon ring 60) until polymer deposition no longer accumulates thereonand the surface of the piece remains free for interaction with theplasma. While this may be practiced as an alternative mode of theinvention, it is not the most preferable mode because: (a) theconsumption of the polymer hardening precursor piece will be higher, and(b) some electrical (RF) coupling to the polymer hardening precursorpiece must be provided to impose the requisite RF bias power on it,complicating its structure. In yet another alternative mode, heating andRF biasing of the polymer hardening precursor piece may be combined.

FIG. 13 illustrates how the embodiment of FIG. 8A may be modified toprovide it with a dome-shaped monolithic ceiling. In particular, theceiling 152, which may have a multiradius dome shape, is hemisphericalin the illustrated embodiment. FIG. 14 illustrates an embodimentcorresponding to FIG. 8B in that the dome-shaped ceiling 152 is dividedinto plural disk and annular sections 152 a, 152 b which areelectrically separate and may be connected to separate RF power sources.FIG. 15 illustrates an embodiment corresponding to FIG. 9 in that thedome-shaped ceiling is combined with the feature of an expendablepolymer-hardening precursor piece in the form of a vertical cylindricalliner 210 adjacent the vertical cylindrical chamber side wall. In orderto maintain the polymer-hardening precursor liner at the requisitetemperature, a heater 215 (such as an electric heater) provides heat tothe liner 210. A temperature sensor 266 monitors the temperature of theliner 210. The output of the sensor 266 is connected to a controller 268governing electric current supplied to the heater 215. The controller268 is of the conventional type which can be programmed with a selectedtemperature. The controller 268 increases or decreases the electriccurrent supplied to the heater 215 depending upon whether the sensor 266senses a liner temperature which is below or above the selectedtemperature, respectively.

In accordance with the methodology of the invention, process uniformityacross the wafer surface is optimized by adjusting the radial densitydistribution across the wafer surface of either or both etchantprecursor species and deposition precursor species in the plasma. Thisis accomplished in the invention by any one or some or all of severalmethods. A first method is to divide the ceiling 152 into pluralradially disposed separate sections (as illustrated in FIG. 8B forexample) and changing the RF power applied to one section relative tothe other. A second method is to change the radial distribution oftemperature across the ceiling. As discussed above, this can beaccomplished by separately controlling the heaters 172 disposed atdifferent radial locations. A third method is to separately adjust theprocess gas flow rate in the radially inner and outer gas flow inlets164 a, 164 b, 164 d.

While the expendable temperature-regulated polymer-hardening precursorpiece has been disclosed in combination with an inductively coupledplasma reactor, other applications are useful. For example, thepolymer-hardening precursor piece may be used in a capacitively coupledplasma reactor. Such an application is illustrated in FIG. 16. In FIG.16 the ceiling 152 is driven by the RF power source 1210 to provideplasma source power into the chamber. No inductive antenna is necessary(e.g., the solenoid antennas 142, 190) although an inductive antenna maybe present even through the reactor is operated in a predominantlycapacitive coupling mode. If there is no inductive antenna over theceiling 152, then it is not absolutely required for the ceiling to be asemiconductor material although such a feature is advantageous in anycase for the reasons described previously herein. If indeed the ceilingis not a semiconductor, the heaters 172 and the associated controlapparatus may be eliminated if desired. In the capacitive coupling mode,the plasma source power is applied predominantly between the ceiling 152and the wafer pedestal 154, so that one of the two RF power sources 1210and 170 may be eliminated as redundant. (In the inductive coupling mode,having two separate sources 1210, 170 is not necessarily redundant sincethe source 170 provides independent RF bias controlling ion energy nearthe wafer while the source 1210 controls the bombardment or activationof the ceiling 152.) FIG. 17 illustrates how the feature of anexpendable polymer-hardening precursor piece in the form of a verticalcylindrical liner 210 on the chamber side wall interior may be combinedwith a capacitively coupled plasma reactor.

FIG. 18 illustrates how the plasma reactor of FIG. 8A may be modified sothat the polymer-hardening precursor ring 63 is replaced by apolymer-hardening precursor chamber liner 63′ near chamber ceiling. Inthis embodiment, the liner 63′ has an outer surface 63′a that fits thesquare corner between the flat ceiling 152 and the cylindrical sidewall150 and an inner surface 63′b which is three-dimensionally shaped andconfines the plasma processing region to the three-dimensional shape. Inthe embodiment of FIG. 18, the liner interior surface 63′b forms atleast a portion of the ceiling of the chamber confining the plasma to avolume whose top has the three dimensional (e.g., dome) shape. In thedrawing of FIG. 18, this three-dimensional shape is a multi-radius domeshape having a radius of curvature which increases from a minimum radiusvalue at the circumferential periphery of the liner 631 to a maximumradius value at the center. However, the shape may be a single-radiusshape or a hemispherical shape. FIG. 19 illustrates how the embodimentof FIG. 18 may be modified to adopt the feature of FIG. 8B in which theceiling 152 is divided into radially inner and outer (disk and annular)portions 152 a, 152 b which are driven with independent RF power levelsand/or RF frequencies.

FIG. 20 illustrates how the embodiment of FIG. 18 may be modified toreplace the solenoid antenna coils 142, 190 with the flat antenna coils50 a, 50 b of the embodiment of FIG. 3. FIG. 21 illustrates how theembodiment of FIG. 20 may be modified to adopt the feature of FIG. 8B inwhich the ceiling 152 is divided into radially inner and outer (disk andannular) portions 152 a, 152 b which are driven with independent RFpower levels and/or RF frequencies.

In the embodiments of FIGS. 20 and 21, the heating and cooling apparatusis distributed across a large proportion of the top surface of theceiling, so that only a relatively small number of windings areaccommodated in the flat coils 50 a, 50 b. In order to increase thenumber of flat coil windings, a simpler heating and cooling apparatusmay be employed in the manner of FIG. 3, in order to accommodate agreater number of flat coil windings.

In each of the embodiments disclosed in this specification in which theceiling 152 is driven with RF power, the RF plasma source power has beendescribed as being inductively coupled by an inductor or inductors, suchas the inner and outer solenoid antennas 142, 190. However, RF plasmasource power may instead be capacitively coupled from the ceiling 152(or ceiling portions 152 a, 152 b) applying sufficient RF power to theceiling 152 with the wafer support or pedestal 154 being connected as anRF return path for the power applied to the ceiling 152, while verylittle or no RF power is applied to the inductors 142, 190 or suchinductors are eliminated.

2. Description of Relating to the Present Invention:

It is now desired to obtain a deeper etch depth at a faster etch ratethan was heretofore attainable. A deeper etch depth is required to meetcontinuing reductions in critical dimension or opening size, while thefaster etch rate is required to meet higher production through-putdemands for lower device cost. It is also desired to improve photoresistfacet selectivity in silicon oxide plasma etching. Such an improvementwould provide a greater margin of safety against removal of silicondioxide material at the edge of an opening due removal (“faceting”) ofphotoresist at the edge. It is further desired to better control theetch profile to obtain a 90% taper of the vertical walls of the etchedopening. It is also desired to critical dimension variance across thewafer surface (e.g., center to edge) to as low as 0.05 microns or lessand to minimize silicon loss at the bottom of each etched opening to 800angstroms or less. As yet another way of reducing costs, it is desiredto reduce the cost of consumable materials in the reactor (such as thefluorine-scavenging precursor material). The present invention describedbelow accomplishes all of the foregoing simultaneously.

For reactors of the type lacking a silicon or silicon-carbide ringaround the pedestal as the fluorine-scavenger precursor material, theonly fluorine-scavenger precursor material is a silicon reactor ceiling.In attempting to address the problem of through-put, the etch rate couldbe increased, theoretically, by reducing the ceiling temperature toreduce fluorine scavenging, thereby increasing the fluorine content ofthe plasma as well as the fluorine content of the protective polymerdeposited on the wafer. A sufficient reduction in ceiling temperaturewould cause more polymer to deposit on the ceiling, thereby reducing thepolymer deposition rate on the wafer. With such changes, the plasmawould etch the silicon oxide layers faster and the high-fluorine contentpolymer itself would etch the silicon oxide layers, the combined effectof which is to significantly increase the rate at which silicon oxide isetched. However, this approach has not seemed feasible because the oxideetch selectivity would decrease due to the increase in fluorine contentof the polymer condensed onto non-oxygen-containing surfaces. A polymerbased upon C—F bonding is weaker than a polymer based primarily uponpure C—C bonding, so that a polymer containing more fluorine is weakerand provides less oxide-to-silicon selectivity. This latter disadvantagecould be addressed by decreasing the plasma source power in order toenhance the oxide selectivity, but it would seem such a decrease wouldreduce the etch rate, so as to offset whatever etch rate gain wasobtained by decreasing the roof temperature.

In meeting all of the foregoing problems, the present invention exploitsthe large effect ceiling temperature has on the etch process. In thetype of reactor having both a silicon ceiling and a silicon (or siliconcarbide) ring, the process characteristics are far more affected by theceiling temperature than the ring temperature, due mainly to the greatersurface area of the ceiling. Thus, for example, it has already beendisclosed in this specification that only the silicon (or siliconcarbide) ring is heated (to on the order of 300 degrees C., for example)while the silicon ceiling is maintained at a much cooler temperature (aslow as 100 degrees C. or even room temperature). In this way, theadvantages of improved polymer quality and etch selectivity obtained byraising the temperature of the silicon ring are at least partly realizedwhile the ceiling temperature is reduced in applying the process of thepresent invention to gain additional advantages, which will be discussedbelow. As for a reactor lacking the heated silicon (or silicon carbide)ring around the pedestal, the process of the present invention includinga reduced ceiling temperature provides additional advantages and thedesired enhancements in performance discussed previously herein.

The low ceiling-temperature process of the present invention includescooling the ceiling sufficiently to (a) reduce the rate at which polymerforms on the wafer by increasing the rate at which it condenses onto theceiling, and (b) increase the fluorine content of the plasma (due to thedecrease in scavenging species taken into the plasma from the ceiling),so that the plasma etches silicon oxide at a significantly faster rate.

In accordance with the preferred embodiment of the present invention,the increase in etch rate and etch depth is accomplished withoutsurrendering other advantages, such as photoresist selectivity,oxide/silicon selectivity, etch profile and center-to-edge etchuniformity. In order to reduce damaging or faceting of the photoresist,the plasma source power (applied to the overhead inductive antennaelements) is reduced. This overcomes the greater tendency of afluorine-rich plasma to attack photoresist. In order to provide greateroxide/silicon etch selectivity, a hydrogen-containing polymer precursorand etchant precursor species, such as CHF₃, is added to theconventional etchant gas, typically C₂F₆. The addition of hydrogen tothe plasma in this manner causes polymer with more C—H or C—H—F bonds toform on the wafer. This type of polymer deposits more rapidly (onnon-oxygen containing surfaces), thereby increasing oxide/silicon etchselectivity. As an additional advantage, a C—H or C—H—F polymer tends toform more strongly on the side walls of etched openings. This improvesthe etch profile, providing the desired wall taper of between 85 degreesand 90 degrees.

Another advantage of adding the hydrogen-containing polymer and etchantprecursor gas (e.g., CHF₃) is that some of the hydrogen disassociatedfrom the added gas forms HF, which promote the silicon dioxide etchrate, thereby further enhancing the process performance.

In one embodiment, another gas is added to the mixture described above,containing a compound of hydrogen and an inert species. One example isHeH₂.

One advantage provided by the greater fluorine content of the plasma(due to the lower silicon ceiling temperature) is that photoresistfaceting is reduced because the greater fluorine content produces moreSi_(x)F_(y) species or radicals in the plasma. Such species emitultraviolet wavelength radiation, which makes chemical changes in theoutermost photoresist layers tending to harden those outer layers. Inthis way, the photoresist acquires a hardened shell which is moredifficult for the plasma to attack, so that there is less faceting, andthe entire photoresist layer is etched uniformly.

The low ceiling-temperature process described here may be used in aplasma reactor of the type illustrated in FIGS. 1 and 2, for example, inwhich plasma source power is applied by a cylindrical inductive coilwound around the chamber side walls and having a silicon (or otherfluorine scavenger precursor material) ceiling, which, as shown in FIG.2, may be heated or cooled. The invention is equally applicable to aplasma reactor of the type illustrated in FIG. 8A in which plasma sourcepower is applied by an inductive antenna overlying the silicon ceiling,where the ceiling can act as a window electrode—being a window throughwhich source power is coupled and an electrode which can be grounded orRF-driven, the reactor further including a heated silicon ring. Asalready described above in this specification, the ceiling can bemaintained at a relatively cool temperature while the silicon ring canbe heated to a relatively hot temperature.

WORKING EXAMPLE NO. 1

A working example of a process of the present invention applied to areactor of the type illustrated in FIG. 2 is as follows: C₂F₆ was fedinto the chamber at rate of between 35 sccm and 15 sccm, CHF₃ was fedinto the chamber at a rate of between 25 sccm and 50 sccm at a chamberpressure of about 17 Mt. The ratio of C₂F₆ to CHF₃ depended upon thedesired etch rate and etch selectivity. HeH₂ was supplied into thechamber at a rate of between 200 sccm and 300 sccm. The resistselectivity improved with greater amount of HeH₂, with little observableeffect on etch rate. 1400 Watts of plasma source power was applied tothe coil antenna, 1100 Watts of plasma bias power was applied to thewafer pedestal. The chamber side wall was maintained at about 160degrees while the ceiling was cooled to 115 degrees C. The etch rate wasfound to lie in a range of about 8,000 to 12,000 angstroms per minutethrough an etch depth of over 2.5 microns.

WORKING EXAMPLE NO. 2

An embodiment of the low ceiling-temperature process of the presentinvention is suitable for use with a reactor of the type illustrated inFIG. 8A for etching a high aspect ratio opening through silicon dioxide.A working example of such a process is as follows: The gas flow rateswere 110 sccm of CHF₃, 15 sccm of CH₂F₂, 400 sccm of Ar. The chamberpressure was maintained near 110 Mt. The silicon ring was heated to 350degrees C. while the silicon ceiling was cooled to 120 degrees C. 790Watts of source power was applied to the inner inductive coil while 2610of plasma source power was applied to the outer inductive coil. An etchrate of about 1.1 micron per minute was achieved, with an etch profilehaving a 90 angle. The photoresist selectivity was about 4.5:1.

WORKING EXAMPLE NO. 3

Another embodiment of the low ceiling-temperature process of the presentinvention is suitable for use with a reactor of the type illustrated inFIG. 8A for etching a bi-level device. A bi-level device has at leasttwo conductive layers, and a single etch step may be required tosimultaneously open contacts to both layers without overetching theupper conductor layer and without underetching the opening to the lowerconductor layer. The process of the present invention can be readilytailored to meet such stringent requirements. An example of such aprocess is as follows: The gas flows were 90 sccm of CHF₃, 10 sccm ofCH₂F₂, 400 sccm of Ar. The chamber pressure was about 88 Mt. 830 Wattsof source power was applied to the inner coil antenna and 2450 Watts tothe outer coil antenna. 1400 Watts of plasma bias power was applied tothe wafer pedestal. The silicon ring was heated to 350 degrees C. whilethe silicon ceiling was cooled to 150 degrees C. The result was that theetch profile had a taper of between 88 and 90 degrees, as desired, theetch rate was about one micron per minute. The photoresist facetselectivity was about 5.5:1.

It was observed that decreasing plasma source power reduced photoresistfaceting but did not improve photoresist etch selectivity. From theforegoing examples, it is seen that the hydrogen-containing polymer- andetchant-precursor gas may be selected from among CHF₃, CH₂F₂ and similargases. The hydrogen-inert gas compound was added only in Working ExampleNo. 1 in the reactor of FIG. 1 (lacking an overhead coil and a heatedsilicon ring), but may be useful in carrying out the invention in areactor of type of FIG. 8A having an overhead coil and a heated siliconring.

Another advantage following directly from the reduction in ceilingtemperature was a proportional reduction in cost of consumables, asignificant advantage of the invention. At the lower ceiling temperaturethe ceiling was worn away at a much lower rate, reducing its frequencyof replacement and thereby reducing the per-wafer cost of consumables.The ceiling must be replaced once its interior surface has been worn bysputtering to the point the process can no longer be easily controlledwithin a desired window. The process of the present invention greatlylengthens the longevity of the ceiling.

In summary, the high plasma density silicon dioxide etch process of theinvention includes cooling the ceiling to 150 degrees C. or below (aslow as room temperature, for example), reducing the plasma source power(e.g., to a range of 1400 Watts to 2500 Watts), maintaining the RF biaspower on the wafer at approximately 1000-1500 Watts, and employing aprocess gas mixture which provides etchant precursor species, polymerprecursor species, hydrogen and an inert gas species. The process isapplicable to either the type of reactor illustrated in FIG. 2 in whichthe plasma source power applicator is a cylindrical inductor woundaround the chamber side wall or the type of reactor illustrated in FIG.8A in which the plasma source power applicator is an overhead inductiveantenna coupling power through the semiconductor ceiling. The lattertype of reactor may further include a disposable heated semiconductorring adjacent the wafer pedestal, which is preferably heated well abovethe polymer condensation temperature (e.g., to 350 degrees C.). Onepreferred process gas providing etchant precursor species, polymerprecursor species and hydrogen is CHF₃. The chamber pressure ismaintained in a range of about 15 mT to 125 mT.

While the invention has been described in detail by specific referenceto preferred embodiments thereof, it is understood that variations andmodifications thereof may be made without departing from the true spiritand scope of the invention.

What is claimed is:
 1. A high plasma density etch process for etching anoxygen-containing layer overlying a non-oxygen containing layer on aworkpiece in a plasma reactor chamber, said process comprising:providing a chamber ceiling overlying said workpiece and comprising asemiconductor material; supplying into said chamber a process gascomprising etchant precursor species and polymer precursor species;providing a plasma source power applicator; supplying RF plasma sourcepower to said plasma source power applicator so as to apply plasmasource power into said chamber; and cooling said ceiling to atemperature sufficiently low to promote polymer deposition thereon. 2.The process of claim 1 wherein said etchant and polymer precursorspecies contain fluorine, and wherein said chamber ceiling semiconductormaterial comprises a fluorine scavenger precursor material.
 3. Theprocess of claim 2 wherein said process gas comprises at least one ofCHF₃ and CH₂F₂.
 4. The process of claim 3 wherein said process gasfurther comprises a non-hydrogen containing etchant and polymerprecursor gas.
 5. The process of claim 4 wherein said non-hydrogencontaining etchant and polymer precursor gas comprises C₂F₆.
 6. Theprocess of claim 3 wherein said process gas further comprises a speciesincluding an inert gas.
 7. The process of claim 6 wherein said speciesincluding an inert gas comprises one of He or Ar.
 8. The process ofclaim 2 wherein providing said plasma source power applicator comprisesproviding an inductive antenna overlying said ceiling, whereby saidceiling is a window to said inductive antenna, said process furthercomprising: applying RF bias power to said workpiece; and controlling anRF potential of said ceiling.
 9. The process of claim 8 whereincontrolling the RF potential of said ceiling comprises one of: (a)holding said ceiling at an RF ground potential; (b) applying an RF biassignal to said ceiling.
 10. The process of claim 8 further comprising:providing a fluorine scavenger precursor material in said chamberseparate from said ceiling; and heating said fluorine scavengerprecursor material to an elevated temperature above a condensationtemperature of a polymer formable from said polymer precursor species ofsaid process gas.
 11. The process of claim 10 wherein said elevatedtemperature is above 170 degrees C.
 12. The process of claim 10 whereinsaid elevated temperature is above 270 degrees C.
 13. The process ofclaim 10 wherein said elevated temperature is near 350 degrees C. 14.The process of claim 10 wherein said heated fluorine scavenger precursormaterial comprises a semiconductor ring concentric with and adjacentsaid workpiece.
 15. The process of claim 10 wherein said heated fluorinescavenger precursor material comprises an interior semiconductor lineradjacent a wall of said chamber.
 16. The process of claim 1 furthercomprising providing a cooling apparatus over said ceiling for carryingout the cooling of said ceiling.
 17. The process of claim 16 whereincooling said ceiling comprises: using plural external semiconductorrings overlying and contacting said ceiling; and using a chilled plateoverlying and contacting said plural external semiconductor rings,wherein applying a plasma source power comprises using inductiveelements overlying said ceiling between ones of said pluralsemiconductor rings.
 18. The process of claim 17 wherein said inductiveelements comprise solenoidal elements.
 19. The process of claim 17wherein said inductive elements comprise coil windings.
 20. The processof claim 1 further comprising maintaining said chamber at a pressurebetween about 15 mTorr and 115 mTorr.
 21. The process of claim 1 whereinproviding a plasma source power applicator comprises: providing pluralrespective inductive elements at respective radial locations overlyingsaid ceiling; and wherein supplying RF plasma source power to saidplasma source power applicator comprises: applying different plasma RFsource power levels to said respective inductive elements to optimizeetch uniformity across said workpiece.
 22. The process of claim 21further comprising providing a cooling apparatus over said ceiling forcarrying out the cooling of said ceiling, comprising: providing pluralexternal semiconductor rings overlying and contacting said ceiling; andproviding a chilled plate overlying and contacting said plural externalsemiconductor rings, wherein said respective inductive elements areprovided so as to overlie said ceiling between adjacent ones of saidplural semiconductor rings.
 23. The process of claim 1 wherein applyingplasma source power comprises inductively coupling source power intosaid chamber.
 24. The process of claim 23 wherein inductively couplingsource power into said chamber comprises coupling power through saidchamber ceiling.
 25. The process of claim 24 wherein providing a chamberceiling comprises providing a silicon-comprising ceiling.
 26. Theprocess of claim 23 wherein said plasma source power applicatorcomprises a coil antenna.
 27. The process of claim 26 whereininductively coupling source power into said chamber comprises couplingpower through a silicon-comprising member.
 28. The process of claim 1wherein providing a chamber ceiling comprises providing asilicon-comprising ceiling.
 29. The process of claim 1 furthercomprising providing at least one of a semiconductor wall or asemiconductor ring.
 30. The process of claim 29 wherein providing achamber ceiling comprising a semiconductor material and providing atleast one of a semiconductor wall or a semiconductor ring comprisesproviding members containing at least one of silicon or carbon.
 31. Theprocess of claim 1 further comprising substantially enclosing saidchamber with a silicon-comprising material.
 32. The process of claim 31further comprising substantially enclosing said chamber with asemiconductor material comprising at least one of silicon or siliconcarbide.
 33. The process of claim 1 wherein said cooling comprisescooling said ceiling to a temperature range at or below about 150degrees.
 34. The process of claim 33 wherein said cooling comprisescooling said ceiling to a temperature range at or below about 100degrees.
 35. A high plasma density etch process for etching anoxygen-containing layer overlying a non-oxygen containing layer on aworkpiece in a plasma reactor chamber, the process comprising: providinga chamber ceiling overlying the workpiece and comprising a semiconductormaterial; supplying into the chamber a process gas comprising etchantprecursor species and polymer precursor species; applying a plasmasource power into the chamber; and providing the chamber with at leasttwo separate sources of fluorine scavenging material, wherein providingthe at least two separate source of fluorine scavenging materialcomprises providing at least two of: a) a semiconductor ceiling, b) asemiconductor wall, and c) a semiconductor ring; and cooling one of theat least two separate sources of fluorine scavenging materialsufficiently to promote polymer deposition thereon while heating another of the at least two separate sources of fluorine scavengingmaterial sufficiently to inhibit polymer deposition thereon.
 36. Theprocess of claim 35 wherein providing the reactor chamber with at leasttwo separate sources of fluorine scavenging material comprises providinga material comprising at least one of: a) silicon or b) carbon.
 37. Theprocess of claim 35 wherein cooling one of the at least two separatesources of fluorine scavenging material further comprises cooling theone of the at least two separate sources of fluorine scavenging materialto within a temperature range sufficiently to promote polymer depositionthereon so as to reduce polymer deposition on the workpiece.
 38. An etchprocess for etching an oxygen-containing layer overlying a non-oxygencontaining layer on a workpiece in a plasma reactor chamber, saidprocess comprising: providing a chamber ceiling overlying said workpieceand comprising a semiconductor material; supplying into said chamber aprocess gas comprising etchant precursor species and polymer precursorspecies; providing inductively coupled plasma source power into saidchamber; and maintaining a temperature of said semiconductor materialwithin a range sufficiently low to promote polymer deposition thereon.